1. Field of the Invention
The present invention relates to a printer, e.g., a stencil printer constructed to transfer ink to a sheet-like recording medium via a master or perforated stencil wrapped around a print drum. More particularly, the present invention relates to the structure of a press roller, which is included in a printer operable in a duplex print mode, for pressing a sheet against a print drum in contact with an image printed on the sheet.
2. Description of the Background Art
A digital stencil printer, extensively used today as a simple printer, includes a thermal head on which fine heating elements are arranged in an array. While a thermosensitive stencil is conveyed, the heating elements are selectively energized in accordance with image data in contact with the stencil for thereby perforating, or cutting, the stencil with heat. After the perforated stencil, i.e., a master has been wrapped around a porous print drum, a press roller or similar pressing means presses a sheet or sheet-like recording medium against the print drum with the result that ink is transferred from the print drum to the sheet via the master, printing an image on the sheet. Various conveying members joining in sheet conveyance, including rollers, belts and pressing members, are arranged on a path extending from a sheet feeding section to a sheet discharging section via a printing section, so that a sheet is conveyed while sequentially contacting such conveying members. It is to be noted that the members, joining in sheet conveyance, include both of conveying members and members that simply steer a sheet in contact therewith.
A current trend in the stencil printers art is toward duplex print mode operation that prints images on both sides of a sheet for reducing the consumption of sheets as well as a space for storage. It has been customary to produce a duplex print by feeding a sheet from a sheet feeding section via a print position to print an image on one side of the sheet, turning the resulting one-sided sheet by hand, and again feeding the one-sided sheet via the print position to print an image on the other side of the sheet. This procedure, however, forces the operator of the printer to turn the one-sided sheet and again set it in the sheet feeding section or neatly position a stack of one-sided sheets, resulting in time- and labor consuming work.
Further, because ink on the one-sided sheet, carrying an image on one or front side thereof, is still wet just after printing, the rollers and press roller disfigure the image if immediately pressed against the sheet for printing an image on the other or reverse side of the same sheet. In light of this, printing on the other side of the sheet has heretofore been effected on the elapse of several hours or more. Particularly, when the image on the front side includes a solid portion, printing on the reverse side has customarily been effected next day because it takes a longer period of time for the solid portion to be dried.
As stated above, the duplex print mode has a problem that a long drying time must be provided between printing on the front side of a sheet and printing on the reverse side of the same sheet, and a problem that the same sheet must be repeatedly passed through the print position two times. Consequently, the duplex print mode needs a net printing time two times longer than a net printing time necessary for a simplex print mode.
To solve the above problem, Japanese Patent Laid-Open Publication No. 2002-103768, for example, proposes a stencil printer operable in the duplex print mode and including a first print drum and a second print drum positioned downstream of the first print drum in the direction of sheet conveyance. First and second pressing means respectively face the first and second print drums via a sheet conveyance path. After the first pressing means has been pressed against the first print drum, the second pressing means is pressed against the second print drum, thereby producing a duplex print.
Japanese Patent Laid-Open Publication No. 9-95033, for example, discloses a stencil printer operable in a duplex print mode and using a single divided master formed with a first and a second image to be respectively transferred to one side and the other side of a sheet. In the duplex print mode, a first press roller presses a sheet against a print drum via the master wrapped around the print drum to thereby print a first image one side of a sheet. Subsequently, the resulting one-sided sheet is again fed by biasing means, so that a second press roller prints a second image on the other side of the sheet.
With either one of the prior art schemes described above, it is possible to produce a duplex print by passing a sheet only one time for thereby substantially halving the printing time necessary for duplex print mode operation to complete.
However, the problem with the schemes taught in Laid-Open Publication Nos. 2002-103768 and 9-95033 is that when an image is to be formed on the other side of a sheet carrying an image on one side thereof, ink deposited on the one side is transferred to the press roller and then transferred to the other side of the next sheet when an image is to be printed on one side of the next sheet, resulting in so-called offset. More specifically, in a stencil printer using emulsion ink, a certain drying time is necessary for ink to be fixed on a sheet. To obviate the retransfer of ink from the press roller to the reverse side of the next sheet, Japanese Patent Laid-Open Publication No. 2002-219849, for example, proposes to form fine projections on the circumference of a press roller included in a downstream printing section.
The press roller disclosed in Laid-Open Publication No. 2002-219849 may be applied to either one of the schemes taught in Laid-Open Publication Nos. 2002-103768 and 9-95033. We, however, experimentally found that when spherical grains substantially identical in shape were so arranged as to form a smooth surface, as shown in FIG. 11(B) of Laid-Open Publication No. 2002-219849, the total area of the grains to contact the image surface of a sheet was not so different from a plane, failing to contribute to the obviation of smearing ascribable to ink retransfer. Even a configuration shown in FIG. 11(C) of the above document increased the area to contact the image surface of a sheet.
While reducing the area over which the press roller contacts the image surface of a sheet may effectively obviate smearing mentioned above, this scheme makes the tips of the portions of the press roller contacting the image area sharp and causes them to penetrate the sheet or even the master via the sheet. The contact area cannot therefore be reduced beyond a certain limit.
We conducted a series of experiments to determine, when a press roller presses one surface of a sheet, carrying an image on the one surface, to print an image on the other surface in the duplex print mode, a time interval after the printing on the one surface that reduces smearing ascribable to retransferred ink. The experiments showed that smearing ascribable to retransferred ink could be almost obviated when the time interval between the printing on the one surface and the printing on the other surface was 5 seconds or more. In practice, however, the time interval should be 3 seconds or less in order to make the most of the merits of a stencil printer.
On the other hand, sheet conveying processes include one that discharges a sheet carrying an image transferred from a print drum, one that again feeds a sheet, carrying an image thereon, in order to print a multicolor image on one side of the sheet, and one that turns a sheet, carrying an image on one side thereof, and again feeds it for producing a duplex print. A sheet is therefore conveyed while sequentially contacting most of members arranged on various paths and joining in sheet conveyance. It follows that reducing the deposition of ink on the press roller or similar rotatable pressing member cannot accurately protect images from smearing.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 5-70010.